Use lua API function nvim_set_hl

Requires nvim 0.7+

This may break existing customizations introduced in #12 lua``` { style = 'bold', }

becomes
lua```
{
    bold = true,
}

Note (from docs):

The fg and bg keys also accept the string values "fg" or "bg" which act as aliases to the corresponding foreground and background values of the Normal group. If the Normal group has not been defined, using these values results in an error.